Project summary 21st International Workshop on HIV and Hepatitis Observational Databases (IWHOD) will take place from March 30th ? April 1st, 2017 in Lisbon, Portugal. The International Workshop on HIV and Hepatitis Observational Databases (IWHOD) has been on-going annually since the mid-1990s. The objective is to bring together researchers from low, middle and high-income countries working on cohorts of patients with HIV to present, in an informal meeting format, new research, and allow open discussions on common issues of cohort methodology, techniques and analyses. One of the particularities of the Workshop is that the data presented, and scientific program, are not made public, papers presented will not yet have been presented at national or international conferences. This and its size (180-190 delegates) have led to an atmosphere of very open debate and discussion, which is particularly helpful to junior, as well as senior researchers. Attendance at the Workshop is open to all, but is contingent on submission and acceptance of abstracts. To allow for the exchange of ideas between young and more senior investigators, senior cohort representative places are also available, which are conditional on abstract submission by colleagues from the respective cohort, thus allowing the work to be presented by young researchers. These places are limited in number, and at a higher delegate fee. The program of the Workshop is based on abstract selection by an international Scientific Committee, chaired by Dr Ole Kirk. The program comprises of six 90-minute sessions, made up of 5-6 papers each, and allowing time for discussion and debate, or a plenary presentation by an expert in the relevant field. Sessions are dedicated to specialist areas including patient management, data to inform public health decisions, and methodology. The 21st IWHOD will feature two plenary talks on current research in hepatitis, and an overview on the field of health economics drawing on examples from HIV research. There will also be a plenary debate on the topic of ageing in HIV research. Poster viewing sessions are an integral part of the meeting. In addition, a number of important side meetings are encouraged, and facilitated, outside of the main program. The Workshop includes researchers from cohort studies from all regions of the globe. Travel scholarships for delegates with accepted abstracts from low and middle-income countries are offered and so are two Young Investigator Awards for early career researchers for best oral and best poster.